In recent years, mobile devices such as cell-phones (and in particular Smartphones), tablets and laptops have become ubiquitous. Such devices commonly include one or two compact digital cameras e.g. a main rear-facing camera (i.e. a camera on the back side of the device, facing away from the user and often used for casual photography) and a secondary front-facing camera (i.e. a camera located on the front side of the device and often used for video conferencing).
The design of many of these cameras is similar to the traditional structure of a digital still camera, i.e. they comprise an optical component (or a train of several optical elements and a main aperture) placed on top of an image sensor (also referred to henceforth simply as “sensor”). The optical component (also referred to as “optics”) refracts the incoming light rays and bends them to create an image of a scene on the sensor.
The dimensions of these cameras are largely determined by the size of the sensor and by the height of the optics. These are usually tied together through the focal length (f) of the lens and its field of view (FOV)—a lens that has to image a certain FOV on a sensor of a certain size has a specific focal length. Keeping the FOV constant, the larger the sensor dimensions (e.g. in an X-Y plane), the larger the focal length and the optics height.
As the dimensions of mobile devices (and in particular the thickness of devices such as Smartphones) are constantly being diminished, compact camera dimensions are becoming an increasingly limiting factor on device thickness. Several approaches have been proposed to reduce compact camera thickness in order to alleviate this constraint. Recently, multi-aperture systems have been proposed for this purpose. In such systems, instead of having one aperture with one train of optical elements, the camera is divided into several apertures, each with dedicated optical elements, and all sharing a similar field of view. Hereinafter, each such aperture, together with the optics and the sensor area on which the image is formed, is defined as a “sub-camera”. Images from the sub-cameras are fused together to create a single output image.
In some multi-aperture camera designs, each sub-camera creates a smaller image on the image sensor compared with the image created by a reference single-aperture camera. Therefore, the height of each sub-camera can be smaller than the height of a single-aperture camera, reducing the total height of the camera and allowing for slimmer designs of mobile devices.
Dual-aperture zoom cameras in which one sub-camera has a wide FOV (“Wide sub-camera”) and the other has a narrow FOV (“Tele sub-camera”) are known. One problem with dual-aperture zoom cameras relates to the height of the zoom Tele sub-camera. There is a significant difference in the height (also known as “total track length” or “TTL”) of Tele (“T”) and Wide (“W”) sub-cameras. The TTL is typically defined as the maximal distance between the object-side surface of a first lens element and a camera image sensor plane. In most miniature lenses, the TTL is larger than the lens effective focal length (EFL). A typical TTL/EFL ratio for a given lens (or lens unit) is around 1.3. In a single-aperture Smartphone camera with a ⅓-¼″ sensor, EFL is typically between 3.5 and 4.5 mm, respectively, leading to a FOV of 70-80°.
Assuming, for example, one wishes to achieve a dual-aperture ×2 optical zoom in a Smartphone, it would be natural to use EFLW=3.5 mm and EFLT=2×EFLW=7 mm. However, without spatial restrictions, the Wide lens will have an EFLW=3.5 mm and a TTLW of 3.5×1.3=4.55 mm, while the Tele lens will have EFLT=7 mm and TTLT of 7×1.3=9.1 mm. The incorporation of a 9.1 mm lens in a Smartphone camera would lead to a camera height of around 10 mm, which is unacceptable for many Smartphone manufacturers.
An example of a solution to the aforementioned problem is described in co-invented and co-owned PCT patent application PCT/IB2014/062180 titled “Dual-aperture zoom digital camera”. Some of the principles of this solution are shown in FIG. 1 schematically illustrating an embodiment of a dual-aperture zoom camera with auto-focus (AF) and numbered 100, in (a) a general isometric view, and (b) a sectioned isometric view.
Camera 100 comprises two sub-cameras, labeled 102 and 104, each sub-camera having its own optics. Thus, sub-camera 102 includes an optics bloc 106 with an aperture 108 and an optical lens module 110, as well as a sensor 112. Similarly, sub-camera 104 includes an optics bloc 114 with an aperture 116 and an optical lens module 118, as well as a sensor 120. Each optical lens module may include several lens elements as well as an Infra-Red (IR) filter 122a and 122b. Optionally, some or all of the lens elements belonging to different apertures may be formed on the same substrate. The two sub-cameras are positioned next to each other, with a small baseline in the 124 between the center of the two apertures 108 and 116. Each sub-camera can further include an AF mechanism, respectively 126 and 128, controlled by a controller (not shown). Camera 100 is “thin” as expressed by TTL/EFL for each sub-camera. Typically, TTLW/EFLW>1.1 and TTLT/EFLT<1.0 (e.g. 0.85).
While the zoom range in camera 100 is about ×2, it would be advantageous to further increase this range. However, this requires increasing further the Tele lens EFL (EFLT), which will cause an increase in the camera height. An increase of EFLT to exemplarily 12 mm will result in an undesirable camera height of for example 0.85×12+0.9=11.1 mm.